This invention relates to inkjet printers, and more particularly to printing systems that include an inkjet printhead. Thermal inkjet printers have experienced a great deal of commercial success since their inception in the early 1980's. These printing systems have evolved from printing black text and graphics to full color, photo quality images. Inkjet printers are typically attached to an output device, such as a computer. The output device provides printing instructions to the printer. These instructions typically are descriptions of text and images to be printed on a print media. A typical inkjet printer has a carriage that contains one or more printheads. The printhead and print media are moved relative to each other to accomplish printing.
The printhead typically consists of a fluid ejecting substrate, which is electrically and fluidically coupled to the printing system. The fluid ejecting substrate has a plurality of heater resistors disposed therein which receive excitation signals from the printhead. The heater resistors are disposed adjacent a plurality of orifices formed in an orifice layer. Ink is supplied to the heater resistors from an ink source affixed to the printhead or from an ink source that is replaceable separate from the printhead. Ink supplied to the heater resistors is selectively ejected, in the form of ink droplets, through the orifices and onto the print media. The ink on the print media dries forming “dots” of ink that, when viewed together, create a printed image representative of the image description. The printed image is sometimes characterized by a print quality metric, which may encompass dot placement, print resolution, color blending and overall appearance such as freedom from artifacts. Inkjet printer manufacturers are often challenged by an increasing need to improve print quality as well as increasing the reliability of the printhead.
The orifice layer and print media are ideally arranged in a parallel orientation to each other. An ink droplet ejected from an orifice in the orifice layer can be represented as a vector that is ideally directed orthogonal to the plane of the print media. Thus, when ink is ejected from the orifice layer of an “ideal printhead,” the difference between where an ink droplet is placed on the print media and where it should have been placed is zero, thus the trajectory error is zero. In actuality, however, variations in the orifice layer manufacturing process result in ink droplets being ejected from an orifice at an angle, which typically ranges between 0 and 2 degrees. These variations in the orifice layer are due to variation tolerances in the orifice formation as well as variation in the planarity of the orifice layer, to name a few.
The effect of trajectory error is exacerbated by separation distance between the printhead and print media. For example, a conventional printhead is separated from the print media by 1.5 mm. If ink is ejected from the orifice layer at an error angle of 2 degrees from the ideal or orthogonal direction, the ink droplet will be displaced 0.052 mm from where it should have been placed on the printing. If, however, the printhead and print media are 0.7 mm apart and ink is ejected at the same 2-degree error angle, the ink droplet will be displaced by only 0.024 mm. This trajectory error tends to reduce or degrade the quality of the printed image because this error affects the positioning of ink on the print media.
The degradation in print quality resulting from trajectory error in conventional printheads is most prevalent where colors of ink are blended to produce “photographic” quality printed images. Here, displaced ink droplets will tend to cause the printed image to appear grainy and streaky. Furthermore, parasitic effects, such as air current, tend to further influence trajectory error of the printing system. These parasitic effects tend to be reduced by lessening the printhead to print media spacing.
The printhead in a typical printing system is separated from the print media by a distance, which may range from 1 millimeter to 1.5 millimeters (mm). This distance between the printhead and print media tends to be limited by the electrical coupling between the fluid ejecting substrate and the printhead body that supports the fluid ejecting substrate. For example, a disposable print cartridge includes a fluid ejecting substrate mounted in a pen body. An encapsulating material is often dispensed on top of the electrical coupling or interconnect to protect or shield the interconnect from ink. Inks used in thermal inkjet printheads tend to have salt constituents that tend to be corrosive and conductive. Once these inks leak into the electrical interface, they tend to produce electrical shorts or corrosion that tend to reduce printhead life. The encapsulant disposed over the interconnect is commonly referred to as an encapsulant bead. The encapsulant bead protrudes beyond the orifice layer of the fluid ejecting substrate and tends to limit the spacing between the printhead and print media. Consequently, there tends to be a limit to the reduction of trajectory error.
In addition to print quality, the printing systems should have high reliability. Two common failure modes that may decrease the reliability of the printhead are: (1) exposure of the interconnect to ink and (2) ink leakage during the shelf life of the printhead. The encapsulant bead may be eroded thereby exposing the interconnect to ink if the printhead is positioned so close to the print media that the encapsulant bead rubs against the print media during printing. The ink tends to corrode the interconnect which ultimately leads to an electrical failure of the printhead, thus making the printhead less reliable.
Conventional inkjet printers employ a cleaning mechanism which includes a wiper that routinely wipes ink residue from the printhead orifice plate. This residue, if sufficient, can either clog the orifices thereby preventing drop ejection or cause misdirected drops. The cleaning mechanism has a predetermined tolerance so that the wiper does not damage the printhead during the cleaning process. However, the wiper tends to be less effective if it is obstructed by a protruding encapsulant bead and could possibly contribute to the erosion of the bead.
A second reliability factor that tends to reduce printhead life relates to environmental conditions that the printhead experiences. Printheads are often exposed to extreme environmental conditions before they are used in a printing system. For example, printheads are often stored in shipping warehouses where temperatures may range from 0–60 degrees Celsius. Or, printheads may be exposed to varying atmospheric pressures during shipping if the printheads are shipped via airplane. In general, conventional printheads are designed to accommodate these extreme conditions without leaking. However, under extreme environmental conditions, as previously described, printheads may leak prior to being used in the printing system. In an attempt to remedy this problem, a tape-like material is placed over the orifice layer to further guard against ink leakage and drying of the ink in the orifices. Ideally, the tape-like material adheres evenly to the orifice layer. However, in conventional printheads, the encapsulant bead previously described may inhibit the tape-like material from uniformly adhering to the orifice layer. If the tape-like material does not uniformly adhere to the orifice layer, ink may leak through the orifice layer and damage surrounding objects. Additionally, ink leaking from the printhead may, over time, harden and clog the orifices as well as contaminate other colors of ink contained within the printhead. Furthermore, leaky printheads are perceived by consumers as being defective and inferior.
Accordingly, there is an ever present need for continued improvements to printing systems that are more reliable and capable of producing even higher quality images. These printing systems should be well suited for high volume manufacturing as well as have a low material cost thus further reducing per page printing cost.